One 3CX800A7 is equivalent to a pair of 8874/8875's in power and load line.
Since the MLA PS wont support a pair of them just remove one and try again
with the circuit as built.
GM3SEK's software is about as good as it gets.
Carl
KM1H
----- Original Message -----
From: "Chris Miller" <c_miller_1@hotmail.com>
To: <TexasRF@aol.com>; <amps@contesting.com>
Sent: Monday, December 31, 2012 9:46 PM
Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit rework
Before I go too much further, a little framing of my background with
tubes. Basically, I have very limited formal training with them directly.
Any work has been repairing to bring back to schematic with tube gear so
far. My training was during the transition from discreet component
transistors to large scale IC and some RF background so I am not totally
at a loss, thankfully. So please feel free to ask what ever questions you
think may be necessary, especially when it comes to the operation of the
tubes.
I was using the value that was shown in the Eimac technical data sheets
for their version of the tube, as that was the tubes in the amp. However,
there is some doubt for me as to whether or not they were referring to one
or two tubes in the typical operation spec area for the sheet. It didn't
say specifically two tubes on this sheet, but on another sheet showed the
class AB area as being two tubes ( that may be helping to lead me astray)
I presumed it was for two tubes as the power shown in the typical specs
exceeded the max ratings area specs, if I was reading it right. If the
specs are for one tube, than I have done my tuning with the resistance
twice what it should be which will throw off everything I did. The
original tube sheets showed a value of 2140 ohms, but again they were not
totally clear on whether they were using two tubes or one in the typical
operation area.
The CW plate current (0.5 A), plate voltage (2200 V) and bias voltage (8.1
V) listed in the spec sheet seemed to line up well with the limits listed
in the Dentron manual, but I imagine the regulation may not be the best,
for either the plate or bias supplies, as it is done old school from what
I have read so far online. (I am going to have to watch the drive closely
with this thing, if I get it working as it should) I am thinking of
putting a board from GM3SEK in it, IF I can get the tank to work as it
should.
BTW, thanks to all who have offered up assistance so far. Everything is
helpful.
Chris
VE3CEA
From: TexasRF@aol.com
Sent: Sunday, December 30, 2012 7:59 PM
To: c_miller_1@hotmail.com ; amps@contesting.com
Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit
rework
Hi Chris, 2700 ohms plate load R seems a bit high. What plate voltage and
current was that figure based on?
73,
Gerald K5GW
In a message dated 12/30/2012 2:49:54 P.M. Central Standard Time,
c_miller_1@hotmail.com writes:
Hi to all,
I have taken on a project that has been bugging me for a bit. I have a
MLA2500 that I picked up that has been retrofitted with a pair of 3cx800
intending to have a transportable amp. From what I have read the tubes
are
supposed to be a close match for the original tubes, as far as tube specs
go. However, I have found that the amp on 160 through 40 has to have the
load control set to 1 to get close to load on power. 20 through 10
appeared
to work o.k.
I decided to sweep the tank through the antenna, with a 2.7k ohm resistor
in
place on the anode to simulate tube load and found that resonances
appeared
off on lower bands. Figuring I had a good handle on the process, I
prototyped up a new tank coil to make mistakes on. I then playing with
tap
points and turns until I achieved what looked like a good set of patterns
for each band, for the range of tuning.
When I attempted to test my results, on dummy load conditions, all bands
are
now showing insufficient capacitance for load on all bands with a little
bit
of range at near max capacity on 10 near the upper end of the band.
Since that seemed to be an epic failure, I must be missing steps in the
process. Is there something I should be watching for or doing so that
what
I see as tank resonance actually lands where it should on real world
testing? Am I now at the point where I have to do some trial additions
of
capacitance to the load control? Is this amp worth the effort to make it
work properly (realizing it is a bit of a sardine can in there)? Has
anyone
else done the retrofit on these amps to the 3cx800 tubes, like this amp
has,
and did the amp still resonate as the original tubes did?
Thanks for any help
Chris VE3CEA
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 10.0.1430 / Virus Database: 2637/5501 - Release Date: 01/01/13
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|